Please, do try before asking for help next time.
In this case, you'll have a very simple pipe:
first, fetch the feed using a fetch feed module,
then, either a regex module as you suggested or a replace string inside a loop module (the loop module allows to apply a non-feed operations (like string manipulation) onto each element of a feed). select item.description in the loop module, then type in _s to be replaced with _b on last occurrence.
Finally, link the loop module to en output module.
You can do even better by having the picture as the media content of the posts, simply using a rename and a regex modules before linking to the output.
In the rename module, from left to right: type-in item.description, copy as, and a dummy field name.
In the regex module your goal is to strip the description of everything except the image url. again, from left to right: item.[the dummy field name used previously], something like ^.*src="([^"]+_b.jpg)".*$ to select everything, but remember the url only, and $1 to replace everything the the url. Then, tick the s box to choose the substitute mode of the regex operation.
For simplicity's sake, use a third module: the create RSS module. it's not necessary to fill-in all field as your feed is already mostly well formatted, but do fill in the media:content section. in url, put the dummy field name, in type, put image/jpeg, the MIME type associated with jpg pictures, and optionally add the desired width and/or desired height.
If you read up to this point, then let me try something. If it works, you can click here to show a pipe link of an example of what you asked, if it doesn't I'll repost.